feat: list backup refs

[dinakars777]

Summary

Users can now find the backup refs created before hard reset without memorizing the internal ref path. git-time-machine --list-backups lists backup refs newest first, shows the commit each points to, and prints exact git show and git reset --hard commands for manual inspection and recovery.

Details

• Add a BackupRef model and GitManager::list_backup_refs() using git for-each-ref refs/git-time-machine/backups.
• Add the --list-backups CLI command, with a non-destructive text output path and a conflict against --export-json.
• Update README, changelog, roadmap, and landing-page copy to describe the backup-ref recovery workflow.
• Add a real-repo regression test covering backup ref lookup and generated recovery commands.

Testing

• cargo fmt --check
• cargo clippy -- -D warnings
• cargo test
• cargo build
• git diff --check
• cargo run -- --help
• cargo run -- --list-backups
• cargo run -- --export-json --list-backups